Generally, exhaust gases emanating from the so-called stationary pollution sources such as boilers and heating furnaces contain ecologically noxious NO.sub.x and, in this respect, constitute origins of air pollution. Establishment of a method capable of effective removal of NO.sub.x from such exhaust gases, therefore, is an urgent need.
Numerous studies have been conducted in search of methods for the removal of nitrogen oxides from exhaust gases and various methods have consequently been suggested. Of these methods, the most promising is the method of selective-contact reduction which causes the NO.sub.x to react with ammonia in the presence of a catalyst and, therefore, converts the NO.sub.x into N.sub.2 and H.sub.2 O which are both harmless ecologically. It is said that the treatment of an exhaust gas by this particular method entails substantially no operational problem where the exhaust gas happens to be a so-called clean gas essentially free from sulfur oxides (hereinafter referred to as SO.sub.x) and dust. In the event that the exhaust gas happens to be a so-called dirty gas containing appreciable amounts of SO.sub.x and dust, however, this method suffers from a serious problem. To be specific, when the exhaust gas under treatment is a dirty gas, the catalyst being used in the treatment is poisoned by SO.sub.x and, consequently, the treatment is prevented from providing desired removal of NO.sub.x satisfactorily over a long period of time. To cope with the difficulty, there has been devised a measure whereby the catalyst so poisoned is moved to a separate place, there to be given a treatment for required regeneration. This measure, however, entails use of facilities complicated in mechanism and expensive to operate. For this reason, various studies have been made with a view to providing a catalyst capable of promoting the reduction of NO.sub.x for a long period of time without adverse affect by the coexistence of SO.sub.x in the exhaust gas under treatment. To this date, no satisfactory catalyst has yet been developed.